Banishing borers

To control iris borers (Macronoctua onusta), the larvae of a moth, destroy their eggs by clearing away leaf litter in fall. In spring, pinch down the tell-tale brown streaks on leaves until you crush the larvae. If the streaking reaches the rhizome, you’ll know where the borer has gone. Dig up the rhizomes and look for exit holes. Cut away the damage and dip the rest in a 10-percent bleach solution for a few minutes to prevent soft rot and drown any larvae still in the rhizomes. Rinse the rhizomes and let them dry off before replanting.

Two beneficial nematodes (Steinernema carpocapsae and Heterorhabditis bacteriospora) are just as good at controlling iris borers as chemical pesticides. Stir them into water and spray them on the plants. The nematodes lay eggs inside the borers’ bodies, and the newly hatched nematodes feed on the borers.